Electroluminescent elements using a luminous body of a material with features of thinness, lightweight, rapid response, low-direct-voltage driving, etc. have been expected to be applied to next-generation flat panel displays. Further, as compared with conventional liquid crystal displays, light emitting devices comprising the electroluminescent elements arranged into a matrix are advantageously viewable at wider angles to have more excellent visibility.
The light emission mechanism of the electroluminescent elements is said to be such that, when an electroluminescence layer is disposed between a couple of electrodes and voltage is applied to the electrodes, electrons injected from a cathode and holes injected from an anode are recombined in the luminescence center of the electroluminescence layer to form molecular excitons, and the molecular excitons are returned to the ground state while releasing energy to emit a light. Singlet excited state and triplet excited state are known as the excited state, and it is believed that the light can be emitted through either excited state.
The light emitting materials, which determine luminescent color of the electroluminescent elements, have variedly been studied. In particular, preferred are materials that can be formed into a film without cohesion upon the film formation to be excellent in film forming properties, have excellent carrier transporting properties, and can emit a light in the solid state.
The luminescent color may be the color in which the above material itself has independently, and may be controlled by a so-called doping method in which the material is used as a host material and doped with a guest material (or a dopant material).
The doping method has a merit that a different luminescent color can be obtained by doping one host material with a different guest material. In the doping method, concentration quenching of the light emitting molecules can be prevented to achieve a high luminance and a high efficiency. Thus, the doping method is effective particularly for red light emitting materials, which are likely to be concentration-quenched. However, at the moment there are few materials that can be suitably used as the host material.
The relation between the host material and the guest material is required to satisfy the basic condition that the HOMO-LUMO level of the guest material is within the HOMO-LUMO gap of the host material. To transfer the energy more efficiently, it is preferred that the maximum emission wavelength of the host be closer to the maximum absorption wavelength of the guest.
However, in red light emission using the conventional host materials, there is a large energy gap between the host material and the red light emitting guest material, so that a problem arises that the energy cannot be efficiently transferred from the host material to the guest material.
For example, tris(8-quinolinolato)aluminum (hereinafter referred to as Alq3) has been widely used as the host material because it can emit a light in the solid state and is excellent in electron transporting properties, film forming properties, and film qualities. However, as reported in Yoshiharu Sato, Molecular Electronics and Bioelectronics, 86-99 (2000) (reference 1), etc., in the case of forming the light emitting layer by codepositing Alq3 and the red light emitting guest material, because Alq3 has a maximum fluorescence wavelength (λmax) of 530 to 540 nm while the red light emitting guest material has a maximum fluorescence wavelength of 560 to 680 nm, the energy is not smoothly transferred and light emission is provided not only by the guest material but also by the host material, whereby it is difficult to obtain the red light emission with an excellent color purity.
It has been reported as a solution of the problem that, by doping with an auxiliary material such as rubrene having a maximum fluorescence wavelength between those of the host material Alq3 and the red light emitting guest material such as DCM2, the energy is smoothly transferred from the host material through the auxiliary material to the guest material and thus only the red light of the guest material can be efficiently obtained (Yuji Hamada, Hiroshi Kanno, Tsuyoshi Tsujioka, Hisakazu Takahashi and Tatsuro Usuki, Red organic light-emitting diodes using an emitting assist dopant, Applied Physics Letters, vol. 75, No. 12, 1682-1684 (1999) (reference 2), etc.)
However, such a method using more materials is disadvantageous in that it is difficult to control the codeposition conditions and that the material loss is increased, thereby requiring further improvement.
Accordingly, an object of the present invention is to provide an electroluminescent element using a material, which is formed into a film without cohesion upon the film formation to be excellent in film forming properties, has excellent carrier transporting properties, emits a light in the solid state, and can be suitably used also as a host material.
Further, another object of the invention is to provide an electroluminescent element using a material, which is used as a host material suitably for efficiently transferring energy to a red light guest material, particularly in case of a red light emission that a maximum absorption wavelength of the guest material is a longer wavelength side.